Composite materials have well-documented advantages over traditional construction materials, particularly in providing excellent mechanical properties at very low material densities. As a result, the use of such materials is becoming increasingly widespread and their fields of application range from “industrial” and “sports and leisure” to high performance aerospace components.
“Prepregs”, comprising a fibre arrangement impregnated with resin such as epoxy resin, are widely used in the generation of such composite materials. Typically a number of plies of such prepregs are “laid-up” as desired and the resulting laminate is cured, typically by exposure to elevated temperatures, to produce a cured composite laminate.
A particular type of prepreg is the so-called “semipreg”, which involves the fibre arrangement being only partly impregnated with resin, leaving a portion of the fibre arrangement in a “dry” state. Semipregs also comprise moulding materials comprising one or more layers of a fibrous reinforcement material which are held in place on a resin layer by the inherent tack of the resin. In these semipregs, the fibrous reinforcement material is unimpregnated or “dry” or it is at least substantially unimpregnated or dry. An example of such a material is disclosed in WO 00/27632.
Semipregs can provide lower porosities in the final cured composite laminate, as the dry regions allow a pathway for entrapped air and volatiles to escape from the laminate.
A common semipreg arrangement is to have a layer of curable resin in contact with one or two adjacent layers of fibres which stay essentially dry with only very little resin migrating into the adjacent fibres. Such semipregs find particular use as part of large load-bearing structures e.g. spars for wind turbine blades.
However, some resin does inevitably migrate into the fibres over time, particularly during storage which compromises the volatile venting properties of the fibrous reinforcement during processing and curing of the prepreg or semipreg. The resin can also pass through to an outer face of the prepreg or semipreg, a phenomenon known as blocking. This is again undesirable as it prevents volatiles from escaping from laminate structures which contain multiple prepreg or semipreg layers.
Processing and curing of the composite material are achieved by increasing the temperature of the material through heating (generally above 40° C.). As the temperature during processing is increased, the resin material starts to cure and the material changes into a solid state.
The curing reaction is an exotherm reaction. To control the reaction, the energy input is typically staged into two or more stages to prevent a runaway exotherm reaction in the moulding which would result in poor mechanical properties of the moulded article. Typically in the first stage of processing, the temperature is increased over time to a desired first level and held constant for a time period (known as the dwell stage or dwell period). Following on from the first dwell period, the temperature is further increased to a second level and held constant over time (second dwell stage). The temperature increase and subsequent dwell may be further repeated depending on the properties of the moulding and the composite material.
Often, in addition to heating and temperature control equipment, the moulding is also pressurized to ensure adequate wetting out of the fibrous reinforcement material by the resin. The material may be pressurized in an autoclave or the material may be in a flexible envelope which is depressurized to increase the pressure on the moulding. This latter technique is commonly known as “vacuum bagging”.
Prepregs and semipregs are typically produced as a roll of sheet material. When it is desired to produce a structure from the prepreg or semipreg the roll is unrolled, and the material laid down as desired. To allow this unrolling to occur even after prolonged storage which could result in migration of the resin into the dry fibrous reinforcement material, a solid non-porous backing sheet, e.g. polythene or paper, is generally applied to an outer face of the prepreg or semipreg.
Upon rolling the material into a roll the backing sheet prevents adjacent layers of prepreg or semipreg from adhering to each other due to any undesirable resin migration. As the roll is subsequently unrolled the backing sheet is then removed from the curable prepreg or semipreg, and is then discarded as waste.
Such solid backing sheets are therefore carefully prepared and formulated to provide a non-stick surface so they can detach from a resinous surface and prevent adhesion between adjacent layers of rolled prepreg or semipreg.
However, this use of the backing sheet is wasteful and adds to the cost of production, as it is discarded after use. Furthermore, there is a possibility that the backing sheet will not be successfully removed from the roll despite efforts to the contrary, particularly when it is applied in an automated process, as is increasingly common. When this happens then the entire stack of prepregs and/or semipregs is rejected.
Attempts have been made to produce semipregs which do not require the use of a backing sheet. This has involved increasing the viscosity of the curable resin layer so that it has less tendency to migrate through any adjacent fibre layer and cause blocking. Whilst this can be achieved, the increase in viscosity reduces the tack and drapability of the semipreg, and only prolongs the inevitable blocking, allowing for a limited storage time. In view of these disadvantages, semipregs without a backing sheet are not currently recommended.
Further improvement in this area would therefore be highly desirable.
The present invention aims to obviate and/or mitigate the above described problems and/or to provide advantages generally.